1. Field of the Invention
The present invention relates to flexible circuit boards and a method for producing the same. Such boards are widely used as connecting devices in electronic devices such as liquid crystal display tubes, ECD and solar cells for mechanically and electrically connecting electrode portions of the electronic devices and of a print circuit board.
2. Description of the Related Art
Recently, flexible circuit boards have been used in electronic devices such as liquid crystal display tubes, ECD and solar cells for mechanically and electrically connecting electrode portions of the electronic devices and a print circuit board. So called anisotropic conductive membrane devices have been widely used for connecting the flexible circuit boards. The anisotropic conductive membrane device is made of a hot melt adhesive containing conductive fine particles. FIG. 1 is a cross sectional view schematically showing an example of such flexible circuit board 20. An adhesive layer 2 is formed on a flexible resistive film 1 and electrical conductive layers 9 are formed on the adhesive layer 2. Each electrical conductive layer 9 is consisting of a conductor 3A of the metal foil having a predetermined pattern and a metal plating layer 5 covering the surface of the conductor 3A. When producing such flexible circuit board 20, a metal foil is adhered to the flexible resistive film 1 with an appropriate adhesive and then processed according to a photoresist technique to provide each conductor 3A with a predetermined pattern as shown in FIG. 1, which is then covered with the metal plating layer 5 to provide each electrical conductive layer 9.
Therefore, as shown in FIG. 1, which is a cross sectional view of the flexible circuit board 20, the electrical conductive circuits 9 are formed as protruding portions and gaps 6 are left between the electrical conductive circuits 9. However, when the electrode portions of the print circuit board and/or the electronic devices and terminal portions of the flexible circuit board 20 are mechanically and electrically connected by means of the hot melt adhesive, adhesive strengths between the electrode portions and the terminal portions are occasionally lowered, because the gaps 6 are formed in the surface of the flexible circuit board 20. Moreover, the conductive fine particles in the anisotropic conductive membrane device occasionally flow into the gaps 6 and electrical resistances between the electrode portions and the terminal portions are increased.